This invention relates generally to temperature control methods and apparatus. This invention relates more particularly to a method and apparatus which allow a controlled temperature to drift up (if the air is being cooled) or down (if the air is being heated) within a predetermined tolerance if a monitored power demand exceeds a predetermined level.
Energy management is critical for environmentally conscious, cost-effective operation of buildings where heating, ventilating and air conditioning (HVAC) are used. Whereas in the past some types of energy management controls have been relatively static (e.g., a residential thermostat set at one desired temperature to be maintained throughout the day in a home), now there are dynamic energy management apparatus that can control HVAC equipment on a highly variable schedule. To enhance the operation of these dynamic apparatus, there is the need for such apparatus to be able to respond to power demand exceeding a predetermined limit by appropriately controlling the HVAC equipment.
By way of an example, it would be desirable if an energy management apparatus that controls HVAC equipment at a commercial building could automatically adjust its control of the equipment when the energy management apparatus detects that the HVAC equipment power demand exceeds a predetermined power demand limit. With this capability the local demand could be limited to avoid a higher "capacity charge."
As another example, an electric utility company that serves thousands of customers may want to send a control signal to one or more of its customers to reduce their respective power demand when the electric utility experiences a peak incidental load condition. With this capability the possibly more drastic action of "load shedding" could be reduced.
Electric utilities can provide a residential customer with a local radio frequency receiver and a drum type ratchet timer for duty cycling the customer's air conditioning compressor. When the timers that have been provided to customers are to be activated (typically based on peak incidental or scheduling for demand limiting), the electric utility transmits the appropriate radio frequency signal. For example, such an activating signal may be sent at 2:00 pm and a deactivating signal may be sent at 8:00 pm. During such an activation period, compressors could cycle "on" for fifteen minutes (for example) if otherwise called for by their respective thermostats, then "off" for fifteen minutes (for example) regardless of what their local systems do. This is a simple means of limiting the utility's summer electrical demand. A disadvantage of this system is that it simply provides an "on" or "off" signal; it functions regardless of what the desired temperature setpoint (e.g., thermostat setting) may be or what the actual controlled temperature becomes. Thus, the activated drum timer cycles on and off the same regardless of ambient conditions; therefore, during days of high outside temperatures, residential home temperatures could rise to uncomfortable levels. For example, during a one hour period while the drum timer is active, the air conditioning compressor runs for at most a cumulative time of 30 minutes and is off for at least a cumulative time of 30 minutes under the foregoing example of fifteen minute duty cycling. With hot outside temperatures and possibly low insulation values in a home, however, it may have been necessary to run the air conditioning compressor for forty-five minutes during the one hour to maintain a comfortable indoor temperature (i.e., if the compressor had not been disabled, the home thermostat would have called for the compressor to run forty-five minutes). If improved control were provided, however, possibly more efficient control could be realized so that both reduced energy load and more reasonable comfort could be obtained.
Thus, there is the need for a temperature control method and apparatus by which both reduced power demand and comfortable temperatures can be simultaneously obtained in response to power demand exceeding a predetermined limit.